look at your bulbs are they turning black on the end? If not, they should still be good. Also do you have just enough like for your tank, not enough or, like most people, overkill on the lighting for better growth?
If you have just right amount, change every year.
Not enough light, change at 6 months.
More than enough light, let them be for 2 years. As the bulbs age they lost their strength, but still put out good light. So if you have plenty of lighting a 96 watt PC may still be putting out only 70 watts after a year, but it still puts out good light. Just a cost saving measure. Electricity is still cheaper in the long run that buying a $50 bulb when the old one still works, just to get a few extra watts.
I have used bulbs up to 4 years!